1. Technical Field of the Invention
The present invention relates to a method for producing a basic amino acid such as L-lysine known as an important additive for livestock feed or L-arginine or L-histidine useful as a drug such as an infusion solution (i.e., parenteral fluid) or the like.
2. Related Art
In a conventionally known method for producing a basic amino acid by means of fermentation, sulfate ions or chloride ions (chlorine ions) have heretofore been generally used as counter anions so as to maintain electrical neutrality of a culture medium. These are supplied mostly in the form of ammonium sulfate as described in, for example, Japanese Patent Application Laid-Open Nos. 30985/""93, 244969/""93, and the like.
Meanwhile, a basic amino acid such as lysine or the like is often sold in the form of the chloride salt (hydrochloride) since it is difficult to crystallize a basic amino acid such lysine or the like in the free state. However, in the production method of a basic amino acid by means of fermentation, since a hydrochloride causes corrosion of a fermentation tank, or the like, a sulfate is often used for fermentation for the purpose of avoiding the corrosion of the tank.
In this case, however, since a basic amino acid product as such resulting from such fermentation is different in counter anion from a basic amino acid product (to be) placed in distribution, the counter anions (such as sulfate ions) are once removed from the basic amino acid salt produced by means of such fermentation, with the use of, e.g., an anion exchange resin and desired counter anions (such as chloride ions) are then added in the form of a free acid, whereby the target basic amino acid salt is produced. Such use of a resin, however, increases loads on environmental protection such as drainage resulting from use of the resin, and the like.
Further, since such use of a resin requires excess acid and alkali, a variety of by-products are also discharged in addition to the target amino acid salt.
In addition, when a basic amino acid such as lysine or the like is to be placed in distribution in the form of a solution-type amino acid feed additive, the solubility of the amino acid in the feed additive solution decreases due to the presence of counter anions, if present therein, so that the counter anions must be removed with the use of a resin in order to obtain an amino acid solution with a high concentration.
In the case of lysine as an example, lysine hydrochloride can be dissolved in water at 10xc2x0 C. in an amount of at most 43 g in terms of lysine per 100 g of water, and lysine xc2xd sulfate can be dissolved in an amount of at most 68 g per 100 g of water. On the other hand, in the case of a solution having only lysine dissolved therein (a free lysine solution), the solution is alkaline in nature, and lysine can be dissolved therein in an amount of as much as 120 g per 100 g of water. In this connection, refer to Japanese Patent Application Laid-Open No. 256290/2000.
As could be understood from the above, removal of the counter anions from a basic amino acid solution is essential or indispensable to prepare a basic amino acid solution having a high concentration.
There has been known as a conventional method for purifying an amino acid fermentation broth with the use of an ion exchange membrane, a method (as disclosed in Japanese Patent Publication No. 7666/1960) in which the amino acid moiety in an aqueous solution of an amino acid salt is caused to pass through the ion exchange membranes with the use of an electrodialyser equipped with a plurality of cation exchange membranes and anion exchange membranes, the two kinds of ion exchange membranes being disposed alternately, whereby the amino acid is produced. The performance of the method, however, is not necessarily high in terms of electrical efficiency due to low mobility of organic molecules such amino acid or the like.
Further, in the case of a solution which contains a large amount of various organic metabolites and the like resulting from microbial fermentation, which, in turn, cannot pass through a cation exchange membrane and an anion exchange membrane, their concentrations become so significantly high at the surfaces of the ion exchange membranes that these organic metabolites are deposited or agglomerated and eventually accumulated on the surfaces of the ion exchange membranes to clog the membranes, which ends in making a continuous operation impossible disadvantageously.
[Problems to be Solved by the Invention]
It is an object of the present invention to provide a method for obtaining a basic amino acid solution having a high concentration within the concentration range in which crystals of a basic amino acid salt are not deposited, by removing the counter anions from a solution of the basic amino acid salt efficiently by use of electrodialysis.
[Means for Solving the Problems]
The present inventors have made extensive and intensive studies to achieve the above object and found that, in removing the counter anions of a basic amino acid by means of electrodialysis, when an alkali aqueous solution is added to a solution of the basic amino acid salt (solution to be subjected to electrodialysis) during the electrodialysis, the counter anions of the basic amino acid such as sulfate ions or the like can be efficiently removed to such degree that they remain in an amount of 40 mol % or smaller based on the amino acid. The present invention has been completed on the basis of these findings.
Accordingly, the present invention relates to a method for producing a basic amino acid solution which comprises subjecting a solution of a basic amino acid salt to electrodialysis with the use of an electrodialyser equipped with cation exchange membranes and anion exchange membranes in combination, wherein an alkali aqueous solution is added to the solution of a basic amino acid salt during the electrodialysis, whereby not only desalting is caused but also the counter anions of the basic amino acid are removed to such degree that the said counter anions remain in an amount of 40 mol % or smaller based on the basic amino acid, and also to a method for producing a basic amino acid solution which comprises subjecting a solution of a basic amino acid salt to electrodialysis with the use of an electrodialyser equipped with anion exchange membrane alone, wherein an alkali aqueous solution is added to the solution of a basic amino acid salt to adjust the pH of the solution to 7 to 10 during the electrodialysis, whereby the counter anions are removed.